The Thuidiaceae contain about 16 genera and 150 or more species widely distributed in temperate and tropical regions. Three or four genera and about 30 species are recorded for the Neotropics. Touw (2001) has provided the most recent review of the family based on a reevaluation of morphological features of the Thuidiaceae. One of the more important conclusions is that Cyrto-hypnum, previously advanced by Buck and Crum (1990) should be subsumed under Pelekium, and Haplocladium retained in the Thuidiaceae (presently not followed here). The distinction between the Leskeacaeae and Thuidiacae is at best vague, and the latter family probably should be subsumed under the former. Pelekium (or Cyrto-hypnum) and Thuidium are in great need of critical study in the Neotropics; the status of a significant number ofdescribed species is unknown. Tentatively the keys and comments for Pelekium and Thuidium given below are adapted and modified in part from Buck (1998) and Touw (2001); in a few cases from Gier (1980). For the study of Thuidiaceae attention must be given to branching pattern, i.e., 1-, 2-, 3-pinnately branched; paraphyllia are needed, best observed both on stems with some of the leaves removed and separated from stem, to examine the surface ornamentation; stem and branch leaves are require with particular attention given to papillae, together with paraphyllia (branching pattern and degree and position of papillae), in a few cases seta ornamentation and the disposition of the capsule are required, particularly for Pelekium.